Cardiac output, i.e., the amount of blood ejected from the left ventricle into the systemic circulation per minute, is one of parameters which is very important for controlling the condition of a cardiovascular system of a human being or any mammal. This parameter is important in anesthesiology, intensive care, during surgery, and in similar situations where continuous control of cardiac activity of a patient is critical.
At the present time the existing methods for measuring cardiac output may be divided generally into invasive methods and noninvasive methods. Each of the above categories, in turn, can be classified on the basis of the principles on which the methods are based upon. For example, invasive methods may be subdivided into thermodilution methods, dye dilution methods, etc. Among these, thermodilution methods for measuring cardiac output are the most simple and accurate methods which, therefore, find most frequent application in the field of hemodynamics. In accordance with these methods, a cardiac catheter, e.g. a Swan-Ganz type catheter, is introduced into the lung artery and is used for measuring the cardiac output (see an article by J. Conway and P. Lund-Johansen in "European Heart Journal", 1990, 11, p. 17). Measurements by thermodilution methods have an accuracy within the range of 10-15%. In measuring parameters of hemodynamics, thermodilution is considered by clinicians to be a "Gold Standard", i.e. a generally accepted way of making hemodynamic measurements.
However, a disadvantage of this method, as well as any other invasive methods, is that it requires an introduction of a catheter into the cardiovascular system of a patient. It is known that long dwelling of the catheter in the patient's body may lead to complications. Another disadvantage of all invasive methods is their high cost and requirements for highly trained personnel. Thermodilution, e.g., as it is commonly practiced, provides discrete measurements, rather than continuous monitoring of cardiac output.
Noninvasive methods of making hemodynamics measurements, such as cardiac output, etc., in general, can be divided into ultrasound methods, echocardiography methods and impedance methods, etc.
Typically,an impedance method is carried out with several electrodes which are brought into contact with a part of the patient's body, e.g., with the neck, and/or the abdominal area. Then a high-frequency current (e.g., of about 1 to 5 mA, 50 to 200 kHz) is passed between the electrodes through the body, and the impedance of part of the body is continuously measured. The impedance method is based on the principle that when blood vessels are completely filled with blood during the cardiac impulse, the body has the minimum impedance, and at the moment directly prior to the cardiac impulse, when blood vessels have minimum volume of blood, the impedance of the body is at its maximum. The difference between the maximum and minimum impedances is proportional to a heart stroke volume (hereinafter referred to as "stroke volume"). The stroke volume is the volume of blood ejected by the left ventricle during a single systole. After appropriate calibration, an absolute value of stroke volume is obtained (S. W. White, et al. European Heart Journal, 1990, 11, p. 79).
A drawback of the impedance method is that when carried out simultaneously with the introduction of various substances into the patient's body which possess electrolytic properties (e.g., a physiological solution) the measurement is inaccurate. Similar inaccuracies occur with the loss of blood. Furthermore, the impedance method is inconvenient and expensive because it involves a complicated procedure of installating sensors over the patient's body.
In another noninvasive method, stroke volume may be measured in terms of the duration of a sphygmic phase and heart rate. See Leshchenko A. I. in Russian Journal "Vrachebnoe Delo", 1973, No. 12, pp. 28-30. The sphygmic phase is a period of time from opening to closing of the aorta valve. Heart rate is the number of ventricular contractions per minute. In accordance with this method, the stroke volume is calculated by means of the following equation: EQU SV=a.multidot.T+b.multidot.HR-c,
where a, b, and c are constants which are determined experimentally, T is the duration of the sphygmic phase, and HR is the heart rate. The accuracy of measurement with this method, as well as all other known noninvasive methods, has been found insufficient for widespread, practical application.